Plastic lumber is currently produced from high-density polyethylene (“HDPE”). This material is ubiquitous—it is the predominant material found in non-soda bottle applications in the U.S. and can be obtained from curbside collection programs. Not surprisingly, therefore, HDPE has become an acceptable substitute for chemically treated wood in many commercial products.
If HDPE has a shortcoming in terms of performance, it is its reduced stiffness compared to wood. Standard yellow pines have a modulus (a measure of stiffness) as measured along the main axis, of 1 to 1.2 million psi. Modulus tends to decrease with duration of exposure to the environment. In the case of yellow pine, for example, over the course of 10 years of outdoor exposure, its modulus will drop by 25-50%. The modulus of HDPE-based plastic lumber is typically about 10% of new standard yellow pine. In contrast to wood however, the modulus of this type of lumber does not significantly diminish over the same period of time.
To increase the stiffness of HDPE in railroad ties, materials such as fiber-reinforced composites, polymer-fiber-reinforced composites, mineral filled polymer composites, hybrid plastics, steel and concrete composites, have been added. See U.S. Pat. Nos. 5,789,477, 5,916,932 and 5,298,214. These efforts have been moderately successful, at best. It has been found to be unpredictable as to which stiffening agents can be mixed with HDPE, or any other plastic, and produce a material that is cohesive, workable and that meets the necessary performance criteria, all in a cost effective manner. For example, polyvinylchloride (PVC) and polyethylene terephthalate (PET) are unworkable at conditions in which HDPE is typically processed.
In addition to identifying materials that are compatible with HDPE from these standpoints, there is recognition of the benefit of using recycled materials. This would ensure a plentiful supply of material at low cost while providing an environmentally friendly solution to waste management.